In 1984 the American National Standards Institute (ANSI) T1 Committee promulgated the Synchronous Optical NETwork (SONET) standard in ANSI document T1.105. The SONET standard essentially added new digital data rates to the North American digital hierarchy above the digital data rate of 44,736,000 b/s (44.736 Mb/s), which, at the time, was the highest digital data rate deployed. The new SONET digital data rates began at approximately 52 Mb/s, the STS/OC-1 rate, and extended up to approximately 2,500,000,000 b/s (2.5 Gb/s), the STS/OC-48 rate, i.e., 48 times the STS/OC-1 rate. Since that time ANSI has further extended the North American digital hierarchy up to approximately 10 Gb/s, STS/OC-192. Although discussions have taken place in the ANSI committees regarding increasing the SONET digital hierarchy up to 40 Gb/s, an increase to 40 Gb/s may be years away because of economic and technical issues. Thus, 10 Gb/s may be a real upper bandwidth limit for some time to come up. In addition, SONET transport systems are limited to fiber spans having maximum lengths of 40 kilometers (40 km) or 25 miles.
Despite the availability of these relatively large bandwidth transport pipes network service providers have nonetheless been demanding the capability to carry even larger amounts of data over fiber spans having a reach of more than 25 miles. The higher demand is primarily attributable to the accelerating pace of traffic growth in the inter-exchange carrier networks. The traffic growth has been dominated by the growth in data traffic. This data traffic growth is also beginning to creep into the local exchange network causing similar network exhaustion problems. Fiber exhaustion in the local exchange network may also be attributed to the increasing demand for internet access, and support of broadband technologies such as Digital Subscriber Loop and 100 Mb/s Ethernet. In any event, the higher traffic demand has exhausted the capacity of the inter-exchange and local interoffice fiber plant at a time when new fiber installations are effectively prohibited by cost and right of way concerns.
Wavelength Division Multiplexed (WDM) technology has developed as the solution to the fiber plant exhaustion problems described above. WDM systems differ from traditional time division multiplexed fiber systems, e.g., SONET systems, in that in lieu of requiring faster time division multiplexing electronics modulating fast lasers, WDM systems multiplex the individual signals from pulse amplitude modulated systems onto one fiber by assigning each pulse amplitude signal a specific wavelength. For example, several "lower speed input" OC-48 signals may each enter a WDM system as a pulse amplitude modulated 1310 nanometer (nm) laser signal. Each OC-48 signal is then assigned or transposed to a specific wavelength or channel in the 1550 wavelength band, for example, a 1553 nm signal at a rate of at least 2.5 Gb/s. The wavelength conversion is typically performed by a device called a wavelength converting transponder. Each channel or wavelength requires one transponder. After wavelength conversion each individual channel or wavelength is then multiplexed by a wavelength division multiplexer and coupled onto the same fiber. By this method a WDM system may transmit several OC-48 signals onto the same fiber. This results in tremendous cost savings in that each pulse amplitude modulated OC-48 signal transmitted would otherwise require a separate fiber and additional electronics. WDM technology essentially solves the fiber exhaustion and cost problems by increasing the capacity of the fiber without requiring the installation of new fiber. In fact each wavelength carried by a WDM system may be considered a virtual fiber.
WDM systems while providing great cost savings are nonetheless fairly expensive and are proving costly to maintain. One of the major factors contributing to the high maintenance costs of WDM systems is the fact that current WDM systems require a different transponder for each channel. Thus, a service provider has to maintain and track an inventory of incompatible transponders in the event an in-service transponder fails. As such, maintenance costs rise in proportion to the number of channels in a system.
A transponder typically has an optical to electric converter (O/E converter), a laser, and in most cases an external modulator, e.g., lithium niobate modulator. The lower speed input signals entering a WDM system are coupled to the O/E converter on the transponder and converted to an electrical signal. The electrical signal may then be used to directly modulate thc laser on the transponder. On the other hand, for WDM systems having external modulators the electrical signal is then used to modulate the external modulator as the continuous wave laser or carrier beam is coupled through the external modulator. The laser on the transponder is typically a distributed feedback (DFB) laser operating in the 1550 nm band. These lasers are required to meet certain guard band requirements critical to the operation of a WDM system. These requirements stem from bandwidth channel limitations. That is, there is some limited range within the 1550 nm wavelength band wherein transmission is feasible. Thus, the more channels the tighter the guard band requirements. For example, ITU Draft Recommendation G.629 recommends a channel spacing of 100,000,000,000 Hz or approximately 0.78 nm for a WDM system having eight or more channels. Furthermore, the laser residing on the transponder has to be stable in order to be kept within the guard band. As such, DFB lasers having narrow linewidths are required. Furthermore, these DFB lasers are usually monitored by costly electronics to maintain their stability. Thus, transponders are costly items.
As such, the more channels and consequently the more transponders employed per WDM system the greater the costs associated with purchasing and maintaining these systems. For example, a forty channel WDM system requires forty transponders, one for each wavelength. Furthermore, a service location having ten forty channel WDM systems would require maintaining and tracking four hundred transponders. Moreover, transponders cannot be shared among different WDM system suppliers. Of great utility, then, would be a generic transponder, able to be placed into operation for any wavelength and capable of being shared among different WDM systems.
Wavelength tunable lasers have been considered as replacements for DFB lasers. Under such a scheme a transponder having a tunable laser could be used for several channels within a WDM system. Tunable lasers, however, cost more than DFB lasers and would require even more costly electronics to be resident on the transponder. Thus, the costs associated with using tunable lasers are currently prohibitive. Furthermore, tunable multi-channel lasers are not yet commercially available.